1. Field of the Invention
The present invention generally relates to mold resin sealed semiconductor devices, and particularly to a semiconductor device sealed with mold resin which is improved not to cause volatility defective or malfunctions of elements.
2. Description of the Background Art
In a conventional resin sealed semiconductor device, with miniaturization of semiconductor elements, there has been a problem that volatility defective or malfunctions of elements are caused due to stresses by the sealing resin. In order to solve the problem, the art is proposed wherein a stress buffering film formed of a resin layer is formed covering surfaces of elements.
FIG. 3A is a plan view of a semiconductor device disclosed in Japanese Patent Laying-Open No. 55-111148 which is the first conventional art and FIG. 3B is a cross-sectional view along the IIIB line in FIG. 3A. Referring to the figures, a silicon dioxide film 3 is formed on a surface of a semiconductor substrate 1. Openings are provided for formation of a base electrode 16 and an emitter electrode 4 in the silicon dioxide film 3. Base electrode 16 and emitter electrode 4 are provided filling the openings. A metal film body 12 is formed surrounding base electrode 16. A bonding wire 8 is connected to emitter electrode 4. Covering emitter electrode 4, base electrode 16 and metal film body 12, a phenylsilicone ladder polymer film 56 which is an undercoat resin layer is provided on semiconductor substrate 1. The phenylsilicone ladder polymer film 56 is formed by applying an anisole solution of phenylsilicone ladder polymer on semiconductor substrate 1. The backside of semiconductor substrate 1 is affixed on a lead frame 11 for external electrode picking-up. The entirety of the semiconductor device is buried in a mold resin 9.
In the above-described conventional device, metal film body 12 is formed for the purpose of preventing that water permeating mold resin 9 degrades the adhesive property between phenylsilicone ladder polymer film 56 and silicon dioxide film 3. With no metal film body 12, since the adhesive property is weak between phenylsilicone ladder polymer film 56 and silicon dioxide film 3, water gets into the interface of these films, which further reaches base electrode 16. As a result, inverse breakdown voltage is produced between base-collector, so that the device cannot effect the function as a semiconductor circuit. Accordingly, formation of metal film body 12 is essential. On the other hand, however, there have been problems that it makes the structure complex and manufacturing process complicated.
Also, the semiconductor device has a problem that its reliability is decreased because, since silicone ladder polymer film 56 is formed on the surface of the semiconductor substrate after connecting bonding wire 8 to emitter electrode 4, there is no stress buffering film on the surface of semiconductor substrate 1 in the processes until that time.
FIGS. 4A-4F illustrate in cross-sectional views steps of manufacturing a semiconductor device provided with ladder type organo-silicone polymer disclosed in Japanese Patent Laying-Open No. 55-50645 which is the second conventional art.
Referring to FIG. 4A, a base region 13 and an emitter region 14 are formed in a surface of a semiconductor substrate 1 (silicon substrate).
Referring to FIG. 4B, a cyclohexanone solution of ladder type organo-silicone polymer (a polymer obtained by heating a mixture of phenyltriethoxysilane and .gamma.-phenylaminopropyltriethoxysilane) is applied with a spinner on the surface of semiconductor substrate 1, which is dried to form an organo-silicone polymer film 106.
Referring to FIG. 4C, a photoresist 7 having openings at portions located above electrode picking-up holes of a collector region 1a, base region 13 and emitter region 14 is formed on organo-silicone polymer film 106.
Referring to FIGS. 4C and 4D, using the photoresist 7 as a mask, organo-silicone polymer film 106 is etched using 1, 1, 1-trichloroethane. Next, removing photoresist 7, a thermal treatment is applied at 350.degree. C. for one hour to make a cross linked ladder type organo-silicone polymer film 106.
Referring to FIG. 4E, an aluminum evaporating film 15 is formed on semiconductor substrate 1 so as to fill openings of organo-silicone polymer film 106, and a photoresist 7 having predetermined shape is formed on aluminum evaporating film 15.
Referring to FIGS. 4E and 4F, using photoresist 7 as a mask, aluminum evaporating film 15 is etched, and removing photoresist 7 subsequently, an aluminum electrode 4 is formed.
According to the above-described second conventional art, since organo-silicone polymer film 106 which is a stress buffering film is formed on the surface of semiconductor substrate 1, the elements are protected in the process. However, in the second conventional art, there have been problems described below.
That is, referring to FIGS. 4C and 4D, 1, 1, 1-trichloroethane is employed for etching organo-silicone polymer film 106. 1, 1, 1-trichloroethane produces hydrogen chloride which is corrosive in a reaction with water, and is easily decomposed with fire to produce gas such as hydrogen chloride. Since 1, 1, 1-trichloroethane has such properties, there have been problems such that it must be handled with great care and selection of materials of an etching device and an etching container is limited. Furthermore, in the second conventional art, although it is made clear that the leak current between source/drain is stable even after leaving the device under circumstances of high temperature, it is not clear if the elements are protected from stresses of mold resin.